In many electronics applications, it is desirable to provide amplification with variable-gain. These applications often include a variable-gain amplifier (VGA, also called a voltage-controlled amplifier). A variable-gain element, like a voltage controlled resistance, can be used to control and/or vary the gain of an operational amplifier, or the like, as a function of a control level (e.g., a control voltage or current). Accurate gain control can be difficult in many environments. For example, simple differential pairs can have manifest gain variation across process corners and operating conditions. Further, many variable-gain implementations include significant switching networks of either load resistors or a switched parallel series of shunt resistors, which can be large and can, themselves, vary with corner.
For large numbers of applications, relatively slow signal paths can manifest smaller operational variations and/or the operational context can tolerate process variations. Further variable-gain amplifiers are often part of larger feedback loops, so that operational variations can be accounted for by the larger circuit path. However, many newer applications include appreciably higher-speed signal paths and smaller component sizes (e.g., transistor sizes on chips, etc.). In such contexts, typical VGA process variations and resulting operational variations can often be too large, and it can be difficult or impossible to meet all specifications for both gain and bandwidth. For example, using fast sampling techniques to accurately control gain in high-speed applications can add too much output loading, which can appreciably reduce bandwidth.